1. Field of invention
This invention relates to circuit protection devices for use in battery protection.
2. Introduction to the Invention
The use of a PTC circuit protection device to protect a battery from overcurrent and overtemperature conditions is well-known. See, for example, U.S. Pat. Nos. 4,255,698 (Simon) and 4,973,936 ((Dimpault-Darcy et al), Japanese Utility Model Application No. 4-75287 (filed Oct. 29, 1992), and International Application No. PCT/JP96/02205 (filed Aug. 6, 1996), the disclosures of which are incorporated herein by reference. In these applications, a device which exhibits a positive temperature coefficient of resistance (PTC behavior) is connected in series with a battery terminal. During normal operation the PTC device is in a low resistance, low temperature condition. On exposure to a very high current which may develop, for example, due to a short circuit, or exposure to a high temperature which may develop, for example, during excessive charging, the device switches into a high resistance, high temperature condition, thus decreasing the current through the battery to a low level and protecting any components in electrical contact with the battery.
PTC devices are commonly installed in battery packs which comprise a plurality of batteries. Such battery packs are designed for use with electrical equipment such as cameras, video recorders, tools, and portable computers. It is desirable to make the battery packs as small and lightweight as possible, but still provide adequate protection in the event of a short circuit. One technique to maximize the use of space in the battery pack is to place the PTC device directly onto the button terminal (i.e. the positive terminal) of the battery. If the device is in the form of a disk or other element with a central hole, the hole can be sized to allow it to be placed over the button terminal. Electrical connection is then made from an electrode on one surface of the disk to the button terminal and from an electrode on the opposite surface of the disk to a second battery. In the design described in Japanese Utility Model Application No. 4-75287 a cap fits over the button terminal and makes electrical contact to it. The PTC device fits over the cap. A second connection is then made from the top surface of the PTC device to a second battery. With this design it is necessary to avoid a possible short circuit between the cap and the inside surface of the central hole of the PTC device and so an insulating ring must be inserted. In addition, if the cap is made large enough to fit easily over the button terminal, it may be too large to actually fit tightly during the life of the battery pack. The design described in International Application No. PCT/JP96/02205 provides a disk-shaped PTC device attached to first and second conductive leads. One of the leads covers essentially the entire surface of the PTC device, including the opening of the disk; the second lead covers part of the opposite side of the disk and extends away from the outer periphery of the disk. The second lead can be bent to make electrical contact to a second battery. In this design, it is necessary to provide an insulating layer to cover the inner periphery of the PTC device so as to prevent electrical shorting of the device by the button terminal. In order to accommodate the insulating layer, the opening of the disk may have to be made larger than is desirable. Furthermore, PTC devices with different size openings are required for different types of batteries.